Light weight high-strength structures are important for aircraft application in hot areas where nickel and cobalt base superalloys are applied. One of the most efficient structures which meets the high strength to weight ratio criteria, along with acoustical dampening, is honeycomb sandwich where a light weight core, made from superalloy foil resistance welded in a honeycomb pattern, is joined to faces made from superalloy sheet. In some cases one of the faces is perforated to allow the structure to act not only as a structural member but also as a sound attenuator. This type of structure is prevalent in nacelles, ducting and tailpipes on aircraft and jet engines. In such applications temperatures over 1000.degree. F are experienced, requiring superalloy construction. Superalloys for this discussion are those alloys based on nickel or cobalt with or without the gamma prime hardening mechanism (i.e., Inconel, Waspalloy, Rene' 41, Hastelloy, Hayne 25, etc.). Brazing has been used in the past to join such structures. However, because of the thin foil used as core, the corrosion resistant aggressive type nickel base braze alloys, containing relatively high percentages of boron and silicon, cause corrosion of the foil and are difficult to employ in production. These braze alloys are characterized in AMS 4775, 76, 77 and 78 specifications. Therefore, such structures are normally fabricated employing less aggressive braze alloys containing high percentages of manganese which limits the temperature range of the structures because of lower strength and corrosion resistance.
Eutectic systems with nickel have been known for many years. There are three basic systems which have been commonly used to obtain nickel based braze alloys with a practical melting temperature; phosphorus, silicon and boron:
______________________________________ phosphorus 11% -- 1616.degree. F silicon 11.5% -- 2106.degree. F boron 4% -- 1969 to 2054.degree. F (depending upon investigator) ______________________________________
Phosphorus is not commonly used as a braze alloy element on superalloys because of high temperature metallurgical problems; therefore silicon, boron, or both, are used primarily to lower the melting point of nickel. Boron is the most aggressive as inferred by the small weight percentage required to obtain a 1969.degree. F eutectic compared with nearly three times as much silicon for a higher eutectic point. Eutectic systems with cobalt based alloys are similar to nickel:
______________________________________ Co--P 11.5% -- 1873.degree. F Co--Si 12.5% -- 2183.degree. F Co--B 5.5% -- 1985.degree. F ______________________________________
Manganese is also used to lower the nickel melting temperature, it being less aggressive than the above eutectic systems. Nickel-manganese forms a minimum (it is not a eutectic) at 1864.degree. F with 60.5 weight percent manganese. Additions of copper with the manganese, employing the minimum reaction of 38.5 Mn - 61.5 Cu at 1568.degree. F, and also silicon are used to alloy less aggressive braze alloys for honeycomb sandwich structures (i.e., Ni, 22.5 Mn, 7 Si, 5 Cu is a common braze alloy).
From those elements which can be used to lower the melting point of nickel and for cobalt, boron is the most desirable in terms of the smallest quantity for the most melting point depression. Its effect upon the functioning of superalloys is also minimal.